Tayfun Babadagli

Wettability alteration: A comprehensive review of materials/methods and testing the selected ones on heavy-oil containing oil-wet systems.

Changing the wetting state of materials is a growing field of research in many areas of engineering and science. In the oil industry, the term wettability alteration usually refers to the process of making the reservoir rock more water-wet. This is of particular importance in naturally hydrophobic carbonates, fractured formations, and heavy-oil systems. This shift in wettability enhances oil recovery in oil-wet and weakly water-wet reservoirs and eventually increases the ultimate oil recovery. For wettability alteration, two methods have been traditionally used: Thermal and chemical. Although many attempts have been made on reviewing the advancement of research in certain aspects of wettability, a comprehensive review of these techniques, especially in terms of the classification of the chemicals used, has been ignored. In this paper, we begin with this review and provide the past experience of wettability alteration in sandstone and carbonate reservoirs. More than 100 papers were reviewed extensively with an in-depth analysis of different methods suggested in literature. The areas of controversy and contradicted observations are discussed. The limitations and the applicability of each method were analyzed. Concerns on up-scaling laboratory findings to field scale are also addressed. The most promising potential methods are identified and their critical conditions highlighted. At the end, a selection of reviewed methods is validated experimentally for one of the most challenging cases: Extra heavy-oil and bitumen recovery from fractured-strongly-oil-wet carbonates. Berea sandstone (aged to be oil-wet) and Indiana limestone samples were saturated with heavy oil (3600cp). Next, the process was initiated by soaking the cores into solvent (heptane or diluent oil) and the oil recovery was estimated using refractive index measurements. Note that solvent was selected to dilute the oil and recover a considerable amount of oil as any chemical or thermal methods yielded inefficiently low recoveries. After the solvent phase, the samples were exposed to wettability alteration through selected chemicals at different temperature conditions through spontaneous imbibition tests to recover more oil and retrieve the solvent diffuse into the sample back. The most promising wettability alteration agents for each type of rock were marked and optimal application conditions (temperatures, injection sequence) were identified. Selected wettability alteration chemicals were finally tested on the bitumen (5-9° API-1,600,000cp) containing Grosmont carbonate sample from Alberta, Canada. It is hoped that this review fills in the gap in the area of wettability alteration processes by summarizing, critically analyzing, and testing the methods suggested in the literature.

Evaluation of EOR methods for heavy-oil recovery in naturally fractured reservoirs

When unfavorable conditions such as heavy-oil, large matrix size, high IFT, low matrix permeability, oil-wet matrix and poorly connected fracture network exist in an oil reservoir, additional support to enhance the matrix oil recovery is inevitable. Water and gas can be injected in order for the matrix oil to be recovered by capillary and gravity forces, respectively. These processes may not yield significant recovery increase as matrix oil becomes heavier and less water wet. In this case, thermal, chemical and miscible gas injection can be thought to overcome the limitations. Recent studies and limited number of field applications have shown that those methods might result in substantial amount of additional oil recovery. On the other hand, there exist certain conditions where the methods may not be technically and economically feasible. This paper reviews different EOR methods and the parameters affecting the performance of heavy-oil recovery based on experimental observations. The application conditions from technical and economical points of view are also discussed.

Temperature effect on heavy-oil recovery by imbibition in fractured reservoirs

Abstract In this study, the effects of temperature on the efficiency of capillary imbibition mechanism were investigated. For this purpose, 3-D capillary imbibition tests under static conditions and at different temperatures (between 20 ° and 90 °C) were conducted using Berea Sandstone samples taken from the same block. In each set of experiments, different types of fluid pairs representing a wide range of oil/water viscosity ratios and interfacial tensions (IFT) were used. Reduction in viscosity and IFT due to temperature increase remarkably altered the capillary imbibition rate. Residual oil saturation was more significantly influenced by the IFT reduction than thermal expansion effect. Also, a study was conducted to investigate the validity of the scaling law when the temperature effect is involved.

Quantification of Natural Fracture Surfaces Using Fractal Geometry

The purpose of this paper is to present an extensive evaluation of the methods to calculate the fractal dimension of natural fracture surfaces. Three methods; variogram analysis (VA), power spectral density (PSD), and roughness-length method (RMS) are applied to 2-D surface data (PSD) and 1-D profiles (VA and RMS) extracted from the surface data of 54 mm diameter crystallized limestone samples. Surface topography of the samples is quantified through a newly designed fully automated device. Before the application, self-affinity of the surface roughness and the applicability of these methods are validated using synthetically generated fractal surfaces. Fractal dimension values of the profiles are obtained as between 1 and 1.5 with a few exceptions. VA and RMS methods yield consistent fractal dimensions while the PSD values are lower than those of the other two methods. In terms of practical applicability, the VA is found more convenient than other two methods because there still exists shortcomings with the PSD and RMS methods due to difficulties in the mathematical analysis of the plots whose slopes are used in the computation of fractal dimension. However, it is observed that the data of limited size fracture surfaces are convenient for fractal analysis and the results are promising for further applications if the fracture surface size is restricted like cores recovered from deep boreholes.

Fractal characteristics of rocks fractured under tension

Fractal geometry can be useful for explaining the fracture behavior and rock properties. The fractal properties of rock fracture surface developed under tension were examined. Seven different rock samples were selected for the tests. An automated surface scanning device was used to map the fractured surfaces. Variogram analysis (VA) (for 1D self-affine sets) and power spectral density (PSD) measurement (for 2D self-affine sets) were applied to calculate the fractal dimension. On a comparative basis, there exists a trend between the fractal dimension and loading rate. The profiles in the loading direction yield higher fractal dimensions indicating the anisotropic feature of fractal. The fractal dimensions obtained by PSD and VA display a relationship for grain size and porosity. Higher porosity samples give different fractal dimensions for upper and lower fractures surfaces.

Selection of proper enhanced oil recovery fluid for efficient matrix recovery in fractured oil reservoirs

As most of the oil is stored in matrix due to its higher storage capacity than fracture network of naturally fractured reservoirs (NFR), reservoir development plans will aim at maximizing the matrix oil recovery. An enhanced oil recovery (EOR) application principally targets (a) to minimize the residual oil in matrix depleting the matrix as effective as possible and/or (b) to accelerate the recovery rate for rapid production of oil cost efficiently. For reservoirs with high recovery factor, minimizing matrix residual oil saturation is a critical issue to extend the life of the reservoir. For reservoirs with low recovery factor, accelerating the production rate is more vital. For each of these reservoir types, different EOR methods should be considered and implemented accordingly. This paper addresses and discusses these two issues and identifies selection criteria for different EOR methods in NFR, namely chemical (surfactant and polymer) and hot water injection. The focus is specifically on matrix type (permeability and wettability), oil and water viscosities, matrix boundary conditions, transfer type (co- or counter-current imbibition), and interfacial tension (IFT). For different values of these properties, the most proper injection fluid type to be used as an EOR fluid is identified to obtain an effective matrix recovery. Co-current and counter-current capillary imbibition experiments at static conditions are conducted to show how effective and how useful these applications are for different rock and fluid types and matrix boundary conditions. For experimentation, strongly water wet Berea Sandstones and oil-wet carbonates (cores from an oil formation) are used. Light crude oil, kerosene and engine oil are selected as the oleic phase. Proper project implementation (adjustment of injection rate and/or concentrations) and selection of the injection fluid for a cost efficient management are also discussed for different conditions outlined above. Finally, the selection criteria of EOR methods based on the rock and fluid properties are defined. This will provide an insight into an effective management of NFRs and reservoir depletion strategies, if the matrix oil recovery is the main target.

Fractal analysis of 2-D fracture networks of geothermal reservoirs in south-western Turkey

Natural fracture patterns of producing geothermal formations in south-western Turkey are mapped at different scales. The fractal dimensions of different fracture network properties, such as spatial distribution, density, connectivity, orientation, and length are measured by different methods. Analysis of the natural fracture patterns from giga to microscales identifies the descending behavior of box-counting fractal dimension with respect to the scale. It is observed that the fracture networks represent scale-invariant properties, but fractal dimensions might notably differ when the mass dimension is measured applying different methods. Anisotropic nature of fracture networks is also included in the fractal analysis.

Influence of intensity and frequency of ultrasonic waves on capillary interaction and oil recovery from different rock types

Oil saturated cylindrical sandstone cores were placed into imbibition cells where they contacted with an aqueous phase and oil recovery performances were tested with and without ultrasonic radiation keeping all other conditions and parameters constant. Experiments were conducted for different initial water saturation, oil viscosity and wettability. The specifications of acoustic sources such as ultrasonic intensity (45-84W/sqcm) and frequency (22 and 40kHz) were also changed. An increase in recovery was observed with ultrasonic energy in all cases. This change was more remarkable for the oil-wet medium. The additional recovery with ultrasonic energy became lower as the oil viscosity increased. We also designed a setup to measure the ultrasonic energy penetration capacity in different media, namely air, water, and slurry (sand+water mixture). A one-meter long water or slurry filled medium was prepared and the ultrasonic intensity and frequency were monitored as a function of distance from the source. The imbibition cells were placed at certain distances from the sources and the oil recovery was recorded. Then, the imbibition recovery was related to the ultrasonic intensity, frequency, and distance from the ultrasonic source.

ON THE APPLICATION OF METHODS USED TO CALCULATE THE FRACTAL DIMENSION OF FRACTURE SURFACES

This paper presents an evaluation of the methods applied to calculate the fractal dimension of fracture surfaces. Variogram (applicable to 1D self-affine sets) and power spectral density analyses (applicable to 2D self-affine sets) are selected to calculate the fractal dimension of synthetic 2D data sets generated using fractional Brownian motion (fBm). Then, the calculated values are compared with the actual fractal dimensions assigned in the generation of the synthetic surfaces. The main factor considered is the size of the 2D data set (number of data points). The critical sample size that yields the best agreement between the calculated and actual values is defined for each method. Limitations and the proper use of each method are clarified after an extensive analysis. The two methods are also applied to synthetically and naturally developed fracture surfaces of different types of rocks. The methods yield inconsistent fractal dimensions for natural fracture surfaces and the reasons of this are discussed. The anisotropic feature of fractal dimension that may lead to a correlation of fracturing mechanism and multifractality of the fracture surfaces is also addressed.

OPTIMIZATION OF CO2 INJECTION FOR SEQUESTRATION / ENHANCED OIL RECOVERY AND CURRENT STATUS IN CANADA

In today’s industrialized world, the generation (and emission) of greenhouse gases (GHG) is likely to continue. The reduction of greenhouse gas emissions through public outreach programs is one approach to mitigate this problem however, in practice, it has not yet been received well by the public due to economic costs. Therefore, permanent storage of these gases in underground reservoirs is believed to be one of the most suitable solutions for the mitigation of greenhouse gases. Sequestration of GHG is not cheap, however, and thus the injection of greenhouse gases into oil or gas reservoirs to enhance production may offset some of these associated costs. The use of CO2 for purely EOR purposes versus injection of CO2 primarily for sequestration are technically two different problems. Proper design practices and technology need to be developed and applied in order to inject CO2 into oil reservoirs not only for the purpose of tertiary oil recovery but also for permanent sequestration. In conventional CO2 injection projects the main purpose is to increase the amount of oil produced per amount of CO2 injected. In contrast optimization of CO2 injection for sequestration purposes requires “maximum oil production with the highest amount of CO2 storage”. Breakthrough time is a critical parameter in this exercise as recycling CO2 is undesirable due to economic and environmental constraints. This paper summarizes on-going research into the conditions that will maximize oil recovery while maximizing the underground sequestration of CO2. Results obtained from numerical modeling of the injection process are discussed. Current efforts on CO2 injection in Canada are also presented.